1. Field of the Invention
This invention relates generally to friction stir welding (FSW) and all of its variations including friction stir processing (FSP), friction stir mixing (FSM) and friction stir spot welding (FSSW) (and hereinafter referred to collectively as “friction stir welding”). Specifically, the invention relates to the problem of tool failure when excessive heat is generated when the tool is used on high melting temperature materials, wherein a collar is used to transfer torque from a rotating shank to a FSW tip, and collar failure results in failure of the FSW tip.
2. Background of the Problems Being Solved
Friction stir welding is a technology that has been developed for welding metals and metal alloys. The FSW process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin. Force is exerted to urge the pin and the workpieces together and frictional heating caused by the interaction between the pin, shoulder and the workpieces results in plasticization of the material on either side of the joint. The pin and shoulder combination or “FSW tip” is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing FSW tip cools to form a weld. The FSW tip can also be a tool without a pin and only a shoulder for processing of another material through FSP.
FIG. 1 is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool 10 having a shank 8, a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized workpiece material. Typically, the pin 14 is plunged into the workpiece 16 until reaching the shoulder 12 which prevents further penetration into the workpiece. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18. In this example, the pin 14 is plunged into the workpiece 16 at the joint line 18.
FIG. 2 is a cross-sectional view of the tool 10. A collar 22 is shown gripping both the shank 8 and the FSW tip 24, wherein the FSW tip is comprised of the shoulder 12 and the pin 14. As the tool 10 is rotated, torque is transmitted from the rotating shank 8 to the collar 22 and then to the FSW tip 24. When the tool 10 is being used on a workpiece that is a high melting temperature material such as steel, the FSW tip 24 is in many situations exposed to temperatures in excess of 1000 degrees C. as it is rotated while traversing steel softened by frictional heating.
Previous state of the art designs that use a collar on a tool for FSW or on a tool in the metal cutting industry use a single collar shrink-fit assembly. In the metal cutting industry such a tool is used for operation of high speed/high stiffness rotary cutting assemblies. The tool in the metal cutting industry relies on a collar that loosens and allows the tool to slip at elevated temperatures so that tool replacement is convenient. Accordingly, prior art designs do not manage the thermal expansion of the collar assembly in order to properly hold the tool above 400 degrees Celsius. The bi-material system of the tools is designed to have a higher thermal expansion rate for the outer collar material, but only for the purpose of easier tool replacement.
Referring to FIG. 1, the frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that may be generally indistinguishable from the workpiece material 16 itself, in contrast to welds using other conventional technologies.
It is observed that when the shoulder 12 contacts the surface of the workpieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the tool pin 14.
During friction stir welding, the area to be welded and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint. The rotating friction stir welding tool 10 provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the base metal, while transporting metal from the leading edge of the pin 14 to its trailing edge. As the weld zone cools, there is typically no solidification as no liquid is created as the tool 10 passes. It is often the case, but not always, that the resulting weld is a defect-free, recrystallized, fine grain microstructure formed in the area of the weld.
Travel speeds are typically 10 to 500 mm/min with rotation rates of 200 to 2000 rpm. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material's thermal properties, high temperature flow stress and penetration depth.
Friction stir welding has several advantages over fusion welding because 1) there is no filler metal, 2) the process can be fully automated requiring a relatively low operator skill level, 3) the energy input is efficient as all heating occurs at the tool/workpiece interface, 4) minimum post-weld inspection is required due to the solid state nature and extreme repeatability of FSW, 5) FSW is tolerant to interface gaps and as such little pre-weld preparation is required, 6) there is no weld spatter to remove, 7) the post-weld surface finish can be exceptionally smooth with very little to no flash, 8) there is no porosity and oxygen contamination, 9) there is little or no distortion of surrounding material, 10) no operator protection is required as there are no harmful emissions, and 11) weld properties are improved. Throughout this document, friction stir welding will be considered to include all processes that can be performed using a friction stir welding tool, including friction stir processing and friction stir mixing.
Previous patent documents have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, metal matrix composites, ferrous alloys such as steel and stainless steel and non-ferrous materials. Another class of materials that were also able to take advantage of friction stir welding is the superalloys. Superalloys can be materials having a higher melting temperature bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.
It is noted that titanium is also a desirable material to friction stir weld. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials.
The previous patents teach that a tool for friction stir welding of high temperature materials is made of a material or materials that have a higher melting temperature than the material being friction stir welded. In some embodiments, a superabrasive was used in the tool, sometimes as a coating.
The embodiments of the present invention are generally concerned with these functionally unweldable materials, as well as the superalloys, and are hereinafter referred to as “high melting temperature” materials throughout this document. Nevertheless, the tool to be taught herein can also be used in less harsh friction stir welding environments when low melting temperature materials are being used.
The present invention is useful for tools being used in many applications, but most especially when performing friction stir processing of high melting temperature materials. Piping and tubing applications require mechanically fastened connectors, often made of dissimilar materials, to maintain a seal over a wide temperature range. As a result, the joint is prone to leaking due to extreme temperature gradients as the two materials expand or contract at different rates. Piping and tubing systems that transport liquid natural gas are one example of mechanical components that must function over an extreme temperature range.
Equipment and machinery having components that operate at elevated temperatures often have rotating components that drive torque to other components. Even when these components are made from materials designed to have high strength at high temperatures, failure of these components during operation can be hazardous and costly. Failure results when components have lowered fatigue resistance and/or reduced creep and rupture strength at these elevated temperatures. In addition, the loads applied to the materials are generally near the yield, fatigue and creep strength of these materials. An example of this problem is a friction stir welding tool used to join high temperature materials such as steels, stainless steels and nickel base alloys, and other high melting temperature materials.